1. Field of the Invention
The present invention relates to a control system in which a plurality of electronic control units perform data communication with one another to drive current consumers under control of the electronic control units, and one electronic control unit used for the control system.
2. Description of Related Art
A plurality of electronic control units (hereinafter, called ECUs) are mounted on a vehicle to control driven members such as current consumers of the vehicle. For example, when a user operates a wiper switch to drive a wiper, a wiper ECU outputs a driving signal to an actuator for driving the wiper. Therefore, the wiper is driven under control of the wiper ECU.
In each ECU having a driving circuit from which a driving current is supplied to a current consumer, a failure sometimes occurs in the driving circuit. For example, when a failure occurs in the driving circuit of an ECU located for outputting a driving current to a current consumer, the ECU cannot control the driving circuit, and the current consumer is not driven normally. To reliably drive the current consumer despite the failure occurring in one driving circuit of the ECU, various techniques have been proposed.
For example, a first patent document (Published Japanese Patent First Publication No. 2004-291943) discloses a technique for driving a wiper motor. In this technique, even when the wiper motor cannot be driven normally under control of a wiper ECU, the wiper motor is driven normally under control of another ECU. More specifically, not only a wiper ECU can control the driving of a wiper according to a control program stored in the wiper ECU, but also an input-output ECU can control the driving of the wiper. This input-output ECU is connected with the wiper ECU through multiple communication lines. In response to a control signal transmitted from the wiper ECU to the input-output ECU through one communication line, the input-output ECU can output electric power to the wiper motor to drive the wiper. Further, other ECUs are connected with one another through the multiple communication lines, and these other ECUs control current consumers other than the wiper.
The wiper ECU judges whether or not a failure occurs in the wiper ECU itself. When the wiper ECU predicts that a failure occurs in the wiper ECU, the wiper ECU transmits a wiper control program stored in the wiper ECU to a backup ECU, this backup ECU stores this program in a non-used area thereof, and the backup ECU performs a wiper control for the wiper. That is, the input-output ECU drives the wiper motor according to a control signal transmitted from the backup ECU.
Further, a second patent document (Published Japanese Patent First Publication No. 2000-148519) discloses a technique for outputting a driving signal to a current consumer through a backup line in response to a failure, occurring in a CPU or a driving circuit located in a driving unit, to drive the current consumer according to the driving signal. More specifically, a multi-communication system has a control unit for outputting data corresponding to user's operation to a communication line, a driving unit with a CPU and a driving circuit for receiving data from the communication line, generating a driving current in the driving circuit according to the data, outputting the driving current to a current consumer to drive the consumer, and an option unit connected with the driving unit through a backup line. The option unit can transmit a driving signal to the current consumer through the backup line to directly drive the current consumer. The option unit monitors the operation of the driving unit. When the option unit detects a failure occurring in the driving unit, the option unit fetches data transmitted from the control unit to the driving unit, generates a driving current in a backup driving circuit according to the data, and outputs the driving current to the current consumer corresponding to the driving unit. Therefore, the option unit directly drives the current consumer through the backup line.
In recent years, the number of ECUs and the number of current consumers mounted on a vehicle have been increased, each ECU controls a single current consumer or a plurality of current consumers, and the ECUs perform data communication with one another through multiple communication lines. Therefore, a system for controlling a plurality of ECUs connected with one another through communication lines is generally located in a vehicle.
When the technique disclosed in the first or second patent document is applied to this control system, one normally-operated ECU can control a current consumer instead of a failed ECU adapted to control the consumer. However, when the technique disclosed in the first patent document is applied to the control system, it is required to additionally provide an input-output ECU for each of ECUs located in the system or to provide an input-output ECU which can receive a control signal from each ECU and can control all current consumers adapted to be controlled by the ECUs. As a result, as the number of ECUs is increased in a vehicle, the size and cost of the control system are considerably increased. Further, as the number of ECUs adapted to transmit control programs to backup ECUs is increased, the volume of a storing area for storing the programs in the backup ECUs is undesirably increased, and the memory capacity and the processing load in the backup ECUs are increased. Therefore, it is not proper to apply the technique disclosed in the first patent document to the control system.
Further, when the technique disclosed in the second patent document is applied to the control system, a plurality of current consumers are controlled normally by ECUs and are controlled by one of backup ECUs in response to a failure occurring in one of the ECUs. In this situation, as the number of current consumers is increased, it is required to increase the number of backup driving circuits and the number of backup lines. Further, the configuration of the option unit is complicated, the size of the option unit is enlarged, and the manufacturing cost of the option unit is increased.